1.3.33

question:

deque. a double-ended queue or deque(pronounced "deck") is like a stack or a queue but supports adding and removing items at both ends. a deque stores a collection of items and supports the following api:
public class Deque<Item> implements Iterable<Item>
            Deque()              //create an empty deque
    boolean isEmpty()            //is the deque empty?
        int size()               //number of items in the deque
       void pushLeft(Item item)  //add an item to the left end
       void pushRight(Item item) //add an item to the right end
       Item popLeft()            //remove an item from the left end
       Item popRight()           //remove an item from the right end
write a class deque that uses a double-linked list to implement this api and a class resizingarraydeque that uses a resizing array.

answer:

import edu.princeton.cs.algs4.*;

public class Deque<Item>
{
    private class Node
    {
        Item item;
        Node left;
        Node right;
    }
    
    private Node head;
    private Node tail;
    private int size;
    
    public Deque()
    {
        head = tail = null;
        size = 0;
    }
    
    public boolean isEmpty()
    {
        return size == 0;
    }
    
    public int size()
    {
        return size;
    }
    
    public void pushLeft(Item item)
    {
        Node node = new Node();
        node.item = item;
        node.left = null;
        node.right = null;
        if(this.isEmpty())
        {
            head = tail = node;
        }
        else
        {
            node.right = head;
            head.left = node;
            head = node;
        }
        size++;
    }
    
    public void pushRight(Item item)
    {
        Node node = new Node();
        node.item = item;
        node.left = null;
        node.right = null;
        if(this.isEmpty())
        {
            head = tail = node;
        }
        else
        {
            tail.right = node;
            node.left = tail;
            tail = node;
        }
        size++;
    }
    
    public Item popLeft()
    {
        if(this.isEmpty())
        {
            return null;
        }
        Item item = head.item;
        Node temp = head;
        head = head.right;
        if(head != null)//防止为空
            head.left = null;
        size--;
        return item;
    }
    
    public Item popRight()
    {
        if(this.isEmpty())
        {
            return null;
        }
        Item item = tail.item;
        Node temp = tail;
        tail = tail.left;
        if(tail != null)//防止为空
            tail.right = null;
        size--;
        return item;
    }
    
    public static void main(String[] args)
    {
        Deque<String> deque = new Deque<String>();
        deque.pushLeft("w");
        deque.pushRight("j");
        deque.pushRight("c");
        StdOut.println(deque.size());
        StdOut.print(deque.popLeft() + "\t");
        StdOut.print(deque.popLeft() + "\t");
        StdOut.print(deque.popRight() + "\t");
        StdOut.println("\n" + deque.size());
    }
}

//忘加迭代器了QAQ

//数组实现可变大小的也没做

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